ENGINEERING ARTICLES

Power transformers connected directly to generators can experience excitation and short-circuit conditions beyond the requirements defined by ANSI/ IEEE standards. Special design considerations may be necessary to ensure that a power transformer is capable of withstanding the abnormal thermal and mechanical aspects that such conditions can create. FIGURE 2.21 Typical simplified one-line diagram for the supply of a generating station’s auxiliary power. Typical generating plants are normally designed such that two independent sources are required to supply the auxiliary load of each generator. Figure 2.21 shows a typical one-line diagram of a generating station. The power transformers involved can be divided into three basic subgroups based on their specific application: 1. Unit transformers (UT) that are connected directly to the system 2. Station service transformers (SST) that connect the system directly to the generator auxiliary load 3. Unit auxiliary transformers (UAT) tha...

There are many different accessories used to monitor and protect power transformers, some of which are considered standard features, and others of which are used based on miscellaneous requirements. A few of the basic accessories are briefly discussed here. LIQUID-LEVEL INDICATOR A liquid-level indicator is a standard feature on liquid-filled transformer tanks, since the liquid medium is critical for cooling and insulation. This indicator is typically a round-faced gauge on the side of the tank, with a float and float arm that moves a dial pointer as the liquid level changes. PRESSURE-RELIEF DEVICES Pressure-relief devices are mounted on transformer tanks to relieve excess internal pressures that might build up during operating conditions. These devices are intended to avoid damage to the tank. On larger transformers, several pressure-relief devices may be required due to the large quantities of oil. LIQUID-TEMPERATURE INDICATOR Liquid-temperature indicators measure the temperat...

When a transformer is taken off-line, a certain amount of residual flux remains in the core due to the properties of the magnetic core material. The residual flux can be as much as 50 to 90% of the maximum operating flux, depending the type of core steel. When voltage is reapplied to the transformer, the flux introduced by this source voltage builds upon that already existing in the core. In order to maintain this level of flux in the core, which can be well into the saturation range of the core steel, the transformer can draw current well in excess of the transformer’s rated full-load current. Depending on the transformer design, the magnitude of this current inrush can be anywhere from 3.5 to 40 times the rated full-load current. The waveform of the inrush current is similar to a sine wave, but largely skewed to the positive or negative direction. This inrush current experiences a decay, partially due to losses that provide a dampening effect. However, the current can remain well ab...

The ability to adjust the turns ratio of a transformer is often desirable to compensate for variations in voltage that occur due to the regulation of the transformer and loading cycles. This task can be accomplished by several means. There is a significant difference between a transformer that is capable of changing the ratio while the unit is on-line (a load tap changing [LTC] transformer) and one that must be taken offline, or de-energized, to perform a tap change. Most transformers are provided with a means of changing the number of turns in the high-voltage circuit, whereby a part of the winding is tapped out of the circuit. In many transformers, this is done using one of the main windings and tapping out a section or sections, as illustrated by the schematic in Figure 2.18. With larger units, a dedicated tap winding may be necessary to avoid the ampere-turn voids that occur along the length of the winding. Use and placement of tap windings vary with the application and am...

THERMAL CONSIDERATIONS The losses in the windings and the core cause temperature rises in the materials. This is another important area in which the temperatures must be limited to the long-term capability of the insulating materials. Refined paper is still used as the primary solid insulation in power transformers. Highly refined mineral oil is still used as the cooling and insulating medium in power transformers. Gases and vapors have been introduced in a limited number of special designs. The temperatures must be limited to the thermal capability of these materials. Again, this subject is quite broad and involved. It includes the calculation of the temperature rise of the cooling medium, the average and hottest-spot rise of the conductors and leads, and accurate specification of the heat-exchanger equipment. VOLTAGE CONSIDERATIONS A transformer must withstand a number of different normal and abnormal voltage stresses over its expected life. These voltages include: Operating vo...

The term load losses represents the losses in the transformer that result from the flow of load current in the windings. Load losses are composed of the following elements. Resistance losses as the current flows through the resistance of the conductors and leads. Eddy losses caused by the leakage field. These are a function of the second power of the leakage field density and the second power of the conductor dimensions normal to the field. Stray losses: The leakage field exists in par ts of the core, steel structural members, and tank walls. Losses and heating result in these steel parts. Again, the leakage field caused by flow of the load current in the win dings is involved, and the eddy and stray losses can be appreciable in large transformers. In order to reduce load loss, it is not sufficient to reduce the winding resistance by increasing the cross-section of the conductor, as eddy losses in the conductor will increase faster than joule heating losses decrease. When the...

Several types of memory are used in computers: Such as Read-and-write memory (RAM), Read-only memory (ROM) and Mass storage. RAM Read-and-write memory (RAM) is used for storing data, instructions, and results during execution of a program. Semiconductor RAM consists of one or more silicon integrated circuits (each of which has many storage cells) and control logic so that information can be transferred into or out of the cell specified by the address. Usually, the information that is stored in RAM is lost when power is removed. Thus, we say that RAM is volatile. Originally, the acronym RAM meant random-access memory, but the term has changed its meaning over time. As the term is used now, RAM means volatile semiconductor memory. (Actually, RAM is also available with small batteries that maintain information in the absence of other power.) The time required to access data in RAM is the same for all memory locations. The fastest RAM is capable of access times of a few nanosecon...